The present invention relates to a fixing device included in a copier, printer, facsimile apparatus or similar image forming apparatus and more particularly to a heat roller included in the fixing device and a method of producing the same.
A fixing device of the type using heat has customarily been included in an image forming apparatus for fixing a toner image formed on a paper sheet, OHP (Over Head Projector) film or similar recording medium. It is a common practice with this type of fixing device to press a press roller against a heat roller. While the press roller and heat roller, which is heated, convey a paper sheet while nipping it, toner on the paper sheet is melted by heat and fixed on the paper sheet thereby.
Today, there is an increasing demand for an image forming apparatus consuming a minimum of power from the environment standpoint. One of power saving schemes proposed in the past is an on-demand system that feeds current to a heater included in the fixing device only when a paper is passed through the device. However, the prerequisite with the on-demand system, which does not effect preheating, is that the surface of the heat roller be immediately heated to a preselected fixing temperature (usually about 180xc2x0 C.) at the time of image formation, i.e., rapid warm-up of the fixing device.
A conventional heat roller includes a hollow metallic pipe accommodating a halogen lamp or similar heater therein. The heater generates heat for thereby heating the entire heat roller. An air layer intervenes between the core or base of the heat roller and the halogen lamp. Because heat is transferred from the halogen lamp to the heat roller by radiation, the air layer lowers heating efficiency. To implement rapid temperature elevation of the heat roller, it has been customary to reduce the wall thickness of the heat roller to 1 mm or below.
A direct heating system, or surface resistance heating system, is another scheme for implementing the rapid warm-up of the fixing device. The direct heating system causes an electric resistance body or similar heat generating layer formed on the outer or the inner periphery of the heat roller via an insulation layer to generate heat. This kind of system can realize a warm-up time shorter than that of the halogen lamp type of system by 20% to 30% because heat is transferred by conduction, as distinguished from radiation.
A heat roller for the direct heating system has been proposed in various forms in, e.g., Japanese Patent Laid-Open Publication Nos. 55-72390 and 7-325497. An insulation layer and a conductive layer have heretofore been formed on such a heat roller by screen printing, adhesion, plating or the like. This kind of method, however, not only needs a long period of time in forming the above layers, but also fails to cause sufficient adhesion to act between the two layers. For example, a procedure for connecting the metallic roller and insulation layer and the insulation layer and heat generating layer by adhesive is time-consuming and high cost. Moreover, air and moisture existing at the interface between the adjoining layers abruptly expand during heating, causing the insulation layer and/or the conduction layer to locally bulge or come off. The portions bulged or come off rise away from the metallic roller and obstruct heat from being released and are therefore heated. This causes the insulation layer and/or the conductive layer to burn.
The problem with the printing scheme or the adhesive scheme is that it is impossible or extremely difficult to form the insulation layer and conductive layer on a heat roller having curvature, irregularities or an irregular shape.
Technologies relating to the present invention are also disclosed in, e.g., Japanese Patent Laid-Open Publication Nos. 8-227245 and 8-262908.
It is therefore an object of the present invention to provide a high quality, low cost heat roller allowing an insulation layer and a conductive layer to be laminated thereon in a short period of time, and a method of producing the same.
In accordance with the present invention, in a heat roller including a metallic roller, an insulation layer formed on the metallic roller, and a conductive layer formed on the insulation layer to serve as a heat generating resistance body, the insulation layer and conductive layer are formed by a roll coater.
Also, in accordance with the present invention, in a method of producing a heat roller including a metallic roller, an insulation layer formed on the metallic roller, and a conductive layer formed on the insulation layer to serve as a heat generating resistance body, the insulation layer and conductive layer are formed by a roll coater.